The proposed research aims at elucidation of the role of torsional flexibility of DNA in protein-nucleic acid interactions. Twisting motions in the DNA helix will be monitored in nanosecond fluorescence depolarization experiments with fluorescent-labeled DNA's. The observed emission anisotropy will be analyzed by comparison with a non-exponential decay law, which was derived by consideration of twisting and bending in wormlike chains: and the torsional rigidity of DNA will be determined. The effects on the torsional rigidity due to perturbations of DNA structure and conformation will be examined, including ionic environment, supercoiling, base composition and sequence, and DNA modifications.